The present invention relates to an electron beam column with at least one aperture element.
Aperture elements are used in electron beam columns to influence beam shape or travel downstream of an electron gun and before electron emission from the column. Such an element typically consists of a body with a passage for part or all of the beam and a blocking surface adjoining an entry opening of the passage. The blocking surface usually lies at least partly in a plane perpendicular to the axis of the passage. When the element is to serve for beam shaping, such as in association with a focussing lens, the passage is calibrated at least at its entry opening and limits the beam to a desired diameter. The blocking surface extends around the entry opening and blocks onward travel of any beam electrons outside that diameter. When the element serves to control travel of the entire beam, such as in a blanking unit operable to provide transient cut-off of the beam, the beam as a whole is deflected away from the entry opening so that all beam electrons impinge on the blocking surface. The deflection is generally always to the same side of the entry opening, but for ease of construction and assembly the blocking surface completely surrounds the entry opening.
A particular problem with an aperture element used for such purposes is the deleterious effect on beam shape and/or orientation resulting from electron departure from the blocking surface. The departing electrons consist of backscattered electrons and secondary electrons, which are impelled back along the column in all directions and give rise to charge locations wherever they impinge in significant concentrations upstream of the element. If a charge location is eccentric with respect to the column axis, as is usually the case and invariably so when the electrons are issued from a blanking unit, the influence of the charge can cause distortion of the beam or deviation from strict coaxiality with the column axis. This then requires corrective measures to restore beam alignment, failing which position errors can arise in the part of the beam emitted from the column. Any such errors are of critical significance, in, for example, an electron beam lithography machine in which the beam generates a writing spot to be positioned with a tolerance of a few nanometres on an electron-sensitive substrate surface. Difficulties of this kind are evident in the column described in FR-A-72 24007, in which the beam entry side of the aperture element has the form of a cone which will deflect stray electrons into the beam itself. The exit side of the element acts as a barrier to a return beam and in part has the form of a cone for directing return beam electrons away from the beam axis, but this latter cone has no influence on electrons at the entry side.
It is therefore the object of the invention to mitigate the effects of electrons scattered from an electron beam aperture element, in particular so as to prevent generation of undesired charge zones liable to act on the beam before entry into the element. Other objects and advantages of the invention will be apparent from the following description.
According to the present invention there is provided an electron beam column comprising a casing, generating means arranged in the casing and operable to generate an electron beam, and at least one aperture element arranged in the casing to be in the path of the beam, the aperture element comprising a body provided with a passage for travel therethrough in a given direction of at least part of an electron beam arriving from the generating means and with a blocking surface adjoining an entry opening of the passage and serving to block travel of part or all of the beam in that direction for the purpose of beam shaping or blanking, characterised in that the blocking surface is arranged to cause departing electrons derived from the blocked beam or part of the beam to be directed at an angle away from the axis of the passage and the aperture element further comprises a screening member having a wall which is co-operable with the blocking surface to bound a trap cavity for the departing electrons and which is arranged to return electrons to the blocking surface for redirection back into the trap cavity.
In the case of such an aperture element the blocking surface directs scattered electrons into the tap space, where the wall of the screening member returns the electrons to the blocking surface for further reflection and generation of scattering electrons. The geometry and relationship of the blocking surface and the wall are preferably such that the electrons remain confined to the trap space. The material of the body between the blocking surface and the boundary surface of the passage itself is sufficient to shield the beam, in the region of its travel through the passage, from influence by the charge building up in the trap space by the confined electrons.
The blocking surface is preferably arranged so that the mean axis trajectory of the departing electrons forms an oblique angle with the axis of the passage at the upstream side of the element with respect to the given direction. The electrons thus have a component of movement in the direction of travel of the arriving beam and accordingly are directed away from travel back towards the beam source. Such an angle of initial departure of the electrons from the blocking surface simplifies structuring of the blocking surface and wall to keep the second and third electron generations within the trap space.
Moreover, the blocking surface, as seen in a section of the body axially of the passage, is preferably rectilinear or concave. Such surface shapes favour geometries by which the stray electrons can be confined to the trap space. If the aperture element is to have purely a blanking function, the blocking surface can be disposed merely at one side of the entry opening, in particular the side to which the beam is deflected for beam blanking. For other uses, and optionally also for blanking use, the blocking surface can entirely surround the entry opening. The surface is then preferably substantially frusto-conical in shape.
The screening member preferably includes a shutter portion extending towards the axis of the passage and terminating at a radial spacing from the entry opening. The shutter portion thus effectively screens part of the blocking surface relative to an arriving electron beam and partially closes off the trap space. The shutter portion is preferably also spaced from the entry opening in a direction opposite to the given direction of beam travel. This simplifies the shaping of the screening member for the purpose of keeping stray electrons in the trap space and increases the volume of the space so as to reduce the charge concentration therein. The shutter portion can terminate in a circular boundary edge, which can be coaxial with, but of greater diameter than, the entry opening of the passage. The diameter is selected to allow travel of all or substantially all of the electrons of the beam to the unscreened part of the blocking surface. The actual diameter will depend on the intended use of the aperture element. If the element is to have a purely blanking function and the blocking surface is disposed only at one side of the entry opening, the boundary edge can be oval or other suitable shape allowing access to the unscreened part of the blocking surface.
The screening member preferably also includes a cylindrical portion connected to the shutter portion and extending substantially parallelly to the axis of the passage. The wall of a screening member thus constructed can, together with the blocking surface, define a trap space in the form of a cavity closed at all sides except for an entrance provided by the boundary edge of the shutter portion.
The blocking surface should, for preference, comprise a material having a low yield of both backscattered electrons and secondary electrons. A suitable material is an aluminium alloy or, to enhance wear resistance and reduce surface oxidisation, titanium. If so desired, this material can be a coating on the body.